This invention is directed to the removal of nitrogen oxides (NOx) from the exhaust gases of internal combustion engines, particularly diesel engines, which operate at combustion conditions with air in large excess of that required for stoichiometric combustion, i.e., lean. It is well known that fuel efficiency improvements in excess of 10% can be achieved in gasoline engines operated at "lean burn" conditions when compared to today's engines which cycle the air to fuel ratio about stoichiometric. Diesel engines, by their nature, operate at lean conditions and achieve 20-30% better fuel economy than stoichiometric gasoline engines.
Unfortunately, the presence of excess air makes the catalytic reduction of nitrogen oxides difficult. Emission regulations impose a limit on the quantity of specific emissions, including NOx, that a vehicle can emit during a specified drive cycle such as an FTP ("federal test procedure") in the United States or an MVEG ("mobile vehicle emission group") in Europe. The regulations are increasingly limiting the amount of nitrogen oxides that can be emitted during the regulated drive cycle.
There are numerous ways known in the art to remove NOx from a waste gas. This invention is directed to a catalytic reduction method for removing NOx. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Two types of catalytic reduction are practiced. The first type is non-selective catalyst reduction (NSCR) and the second type is selective catalyst reduction (SCR). This invention relates to hydrocarbon (HC) lean-NOx reaction which can be either NSCR or SCR.
In the selective catalyst reduction method, a reducing agent or reductant is supplied to the exhaust stream and the mixture is then contacted with a catalyst. A common nitrogen oxide reducing agent typically used in industrial processes is urea or ammonia, which despite the number of prior art automotive patents, is not favored for vehicular applications because of the infrastructure required for reductant sale to the public. Additionally, any SCR method using a separate reducing agent requiring separate on-board holding tanks and environmental provisions (such as provisions to keep the tank from freezing) present difficult engineering problems to overcome.
Perhaps one of the more sophisticated approaches to using urea/ammonia system in a mobile application is disclosed in a series of patents which include U.S. Pat. No. 5,833,932 issued Nov. 10, 1998; U.S. Pat. No. 5,785,937, issued Jul. 28, 1998; U.S. Pat. No. 5,643,536, issued Jul. 1, 1997; and U.S. Pat. No. 5,628,186, issued May 13, 1997. While these patents discuss reducing reagents in a general sense, they are clearly limited to urea/ammonia reductants. According to this system, a catalytic converter having composition defined in the '932 patent, has a reducing agent storage capacity per unit length that increases in the direction of gas flow. This allows for positioning of instrumentation along the length of the catalyst as disclosed in the '536 patent to determine the quantity of ammonia stored in the catalyst. The catalyst is thus charged with the reducing agent such that transient emissions can be converted by the reducing agent stored in the catalytic converter. As explained in the '186 patent, should the vehicle experience a sudden increase in load or acceleration, and without having to wait for an increase in the temperature of the catalytic converter, the stored reducing agent is utilized to reduce NOx, thereby preventing overloading of the catalytic converter, i.e., ammonia breakthrough. The ammonia is metered on/off by the length sensors to "charge" the catalyst with stored ammonia. On acceleration, the metering is stopped to prevent ammonia slip (col. 7, '536 patent).
Urea/ammonia SCR systems are characterized in that the catalysts have the ability to store ammonia at temperatures of the exhaust gases, at least at the relatively low operating temperature ranges of exhaust gases produced by diesel engines. Ammonia SCR systems can therefore be developed by catalyst sizing and ammonia slip control techniques (such as described above) to assure a sufficient quantity of ammonia is present to reduce the NOx emissions generated by the engine and particularly the increased NOx emissions produced, transiently, during engine acceleration or engine load increase periods.
Because of the infrastructure limitations of a urea/ammonia SCR system in mobile applications, there is prior art for the use of hydrocarbons (HC) as a selective reducing reagent for NOx emissions. While reducing catalytic converters (principally base metal zeolites, copper or cobalt ZSM-5 for high temperatures) are able to reduce NOx emissions in the presence of HC at relatively high temperatures (300 to 450.degree. C.), they are not able to store and release the HC at the higher temperatures. It is well known that HC can be adsorbed in zeolite based catalysts at temperatures below 200.degree. C. which are then desorbed at temperatures of about 200.degree. C. or higher (see any number of HC trap patents, for example, U.S. Pat. No. 5,804,155 incorporated by reference herein and SAE paper No. 950747 incorporated by reference herein). There is also prior art for use of HC as a non-selective reducing agent or reductant for NOx emissions. The reducing catalytic converters in this case are precious metal based catalysts and more usually platinum zeolites typically based on Pt ZSM-5 which are active for NOx reduction at lower temperatures (180 to 250.degree. C.).
Because normal operating temperatures of diesel engines produce exhaust gas temperatures above 200.degree. C., it is not usually possible to store and release HC as in ammonia systems. This is a fundamental difference between ammonia based SCR systems and HC based reaction systems.
Despite this fundamental distinction, there is a segment of the prior art that teaches HC can be adsorbed and desorbed (stored and released) at temperatures which include a portion of the normal operating range of the diesel engine. This conclusion appears to be based on the observation that zeolite containing catalysts show better NOx reduction conversion percentages than non-zeolite containing catalysts.
In Mercedes-Benz U.S. Pat. No. 5,935,530, issued Aug. 10, 1999, a three stage catalytic converter is disclosed having an intermediate section which is said to store HC when the engine runs at reduced load and release the stored HC when the engine is at load so that the secondary injection of HC would not have to change in synchronization with the changing engine load (col. 6). The known adsorber catalyst is defined to include a precious metal catalyst. The data disclosed in the '530 patent is based on an artificial gas composition heated at various temperatures and to which a fixed propane/propene ratio is added. The data shows, (as noted in the SAE references), that for a given temperature range, propene-propane will achieve a high NOx reduction. However, there is no evidence that transient NOx emissions can be controlled by this catalyst design.
Johnson Matthey U.S. Pat. No. 5,943,857 issued Aug. 31, 1999, is also directed to a storage of HC, but storage occurring below a temperature range of 190.degree. C. and a desorbtion of the stored HC at temperature ranges stated to be at 198.degree. C. to 200.degree. C. The '857 patent shows NOx reduction levels achieved between catalyst with and without zeolites and shows that zeolite containing catalysts have a higher NOx conversion efficiency than non-zeolite containing catalysts. The '857 patent shows "transient" test data but what is plotted is not the transient NOx emissions during a FTP or MVEG cycle. In a FTP or MVEG cycle, transient emissions occurring during acceleration significantly increase NOx ppm. In the '857 patent, a constant gas mixture is reduced at varying exhaust gas temperatures modeling temperature variations in a drive cycle and the NOx conversion results are plotted. The data shows an overall increase in NOx reduction using a zeolite catalyst. Significantly, even with a constant gas composition, the data shows that temperature changes produce NOx spikes. The '857 patent attributes the spikes to the catalyst heating up (col. 4). While the statement is correct, for reasons discussed in the Detailed Description below, the spikes result from differences in the NOx conversion percentages attributed to the changing temperature. A careful reading of the '857 and '530 patents simply show that reduction levels of NOx can be increased with catalysts containing zeolites which is a known adsorber. Neither patent shows the catalyst is able to store HC similar to the ammonia systems to reduce NOx transient emissions during a regulated drive cycle.
Further, while many arrangements use diesel fuel as the HC source, there are segments within the prior art in which the HC is said to comprise short chain hydrocarbon. For example, Daimler-Benz U.S. Pat. No. 5,921,076, issued Jul. 13, 1999 shows a staged arrangement for injecting i) H.sub.2, ii) H.sub.2 and short chain HC and iii) short chain HC into the exhaust stream as a reductant. Air Products U.S. Pat. No. 5,524,432, issued Jun. 11, 1996 shows methane injection. As late as 1995, assignee's SAE paper No. 950747 recommended low molecular weight HCs with high volatility as the reductant. For reasons which will be discussed below, this invention is limited to long straight chain saturated HCs and unsaturated olefins (of any chain length) which are present in fuel oil or diesel fuel.
It should also be noted that within the diesel fuel SCR prior art a number of arrangements exist for injecting the diesel fuel into the exhaust gas. These include injecting excess fuel into the combustion chamber during the expansion stroke, either through individual injectors or utilization of a common injector rail, and any number of injector designs, including those utilizing pulsation techniques, which dispense fuel oil or diesel fuel into the exhaust gas upstream of the catalytic converter.
There are a number of control schemes or techniques used to control the diesel fuel admitted to the catalyst. For example, Volkswagen's European patent No. EP0881367, dated Dec. 02, 1998, measures residual concentrations of hydrocarbons after the catalytic converter to adjust the reducing fuel oil by a regulating algorithm once certain temperatures have been attained. Similarly, Daimler-Benz U.S. Pat. No. 5,845,487, issued Dec. 8, 1998, uses a nitrogen-oxide sensor after an operating temperature has been achieved to control the system. Unfortunately, applicants have not been able to obtain a commercially acceptable NOx sensor having the response sensitivity needed to control NOx emissions at regulated levels.
Caterpillar U.S. Pat. No. 5,522,218, issued Jun. 4, 1996 illustrates a control methodology typically followed by most HC reducing systems in that certain operating conditions of the engine are mapped and correlated with temperature to perform a mathematical routine, usually by a CPU or the engine's ECU, to determine a quantity of reducing agent which is pulsed metered into the system. Systems which measure engine operating conditions to produce a variable metering of the reductant to the catalyst are generally based upon steady-state engine maps. These systems typically measure or calculate engine speed and/or load, space velocity of the exhaust gas and the exhaust gas and/or catalyst temperature to determine a quantity of NOx produced by the engine and a quantity of HC reductant to be metered into the exhaust gas. Once the operating parameters are known, the quantity of NOx emissions produced and consequently the quantity of HC reductant (amount in addition to HC concentration normally present in exhaust gas) to be metered are known vis-a-vis conventional mapping techniques. (See SAE paper No. 950747 and SAE paper No. 952491.) However, it is well known that transient changes in the operating conditions of the engine, specifically EGR (exhaust gas recirculation) and variable geometry turbocharging (VGT), upon acceleration produce transient NOx emissions which are significant. Those emissions will cause current vehicles to fail future NOx emission standards notwithstanding the fact that such vehicles could meet standards at steady state conditions. In this regard, it must also be noted that response time improvements in microprocessor based control systems have led to improvements in EGR control systems reducing NOx transients. However, the NOx transient is instantaneously formed so a response latency exists in any feedback system. Further, while it is recognized that EGR limits NOx formation by lowering in cylinder oxygen levels, the HC present in EGR systems for diesel engines is limited.
Apart from the prior art segment which erroneously concludes that reducing catalysts can effectively store and release HC reductants at normal engine operating temperatures (discussed at some length above), attempts to account for NOx transient variations in HC reducing systems have been based on temperature sensing systems. Because transient emissions are accompanied by a significant increase in exhaust gas temperatures (see discussion in '857 patent), an early detection of the temperature rise coupled with a reduction of HC added to control NOx may keep the temperature within the NOx temperature reduction window at which SCR zeolite based systems are known to function. While that strategy reduces NOx transient spikes, it must be recognized that the engine instantaneously produces the NOx transient which has flowed through the system before the catalyst temperature has measurably changed. Reference can be had to Toyota's U.S. Pat. No. 5,842,341, issued Dec. 1, 1998, for a discussion of such an approach. The '341 patent discloses a conventional steady state system which measures space velocity and outlet exhaust gas temperature to determine a quantity of fuel oil to be metered to the catalyst. The '341 patent recognizes that transient engine conditions will increase the temperature of the exhaust gas which, in turn, will raise the temperature of the catalytic converter to the point where the temperature "window" at which NOx conversion occurs may be exceeded. To prevent this, exhaust gas temperature is measured upstream and downstream of the catalytic converter and the reductant flow is decreased from the steady-state programmed flow when the upstream gas temperature differential exceeds a set value. By reducing the HC reductant, the exothermic reactions attributed to oxidation of the reactant is reduced and the mass of the catalytic converter will not be heated, at a later time, to as high a temperature as it would be at if the HC reductant were present. The belief is that the NOx temperature window of the reducing catalytic converter will not be exceeded during the engine acceleration and the catalyst will still be able to function. However, there may be insufficient reductant to dispose of the NOx emissions when the HC is reduced. Apart from this, in practice, this strategy will not work under real engine conditions because the temperature increase in the catalyst due to the HC+NOx reaction (or other HC oxidation reactions) lags the flow transient. Therefore, the gas transient has passed through the catalyst before the temperature sensor can call for reduction of the HC flow.